Genistein, a natural isoflavonoid found in soybean products, has been proposed to be associated with a lower rate of breast cancer in Asian women. Studies from our laboratory and others have shown that genistein can induce apoptosis by regulating the expression of apoptosis-related genes in breast cancer cells. However, the precise molecular mechanism(s) by which genistein induces apoptotic cell death is not clear. In order to investigate such mechanism, we tested the role of Akt and NF-kappaB in genistein-treated MDA-MB-231 breast cancer cells. We found that inhibition of cell growth and induction of apoptosis by genistein are partly mediated through the downregulation of Akt and NF-kappaB pathways. Gel shift assay showed that NF-kappaB DNA-binding activity in MDA-MB-231 cells transfected with Akt cDNA was induced, suggesting that there is a cross-talk between NF-kappaB and Akt signaling pathway. Moreover, we found that genistein could abrogate EGF and Akt induced NF-kappaB activation. From these results, we conclude that the inactivation of NF-kappaB by genistein in MDA-MB-231 breast cancer cells is partly mediated via Akt pathway, which could be useful for rational design of strategies for the prevention and/or treatment of breast cancer.